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Date:   Mon, 27 Jun 2022 14:16:58 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc:     "frederic@kernel.org" <frederic@kernel.org>,
        "joel@joelfernandes.org" <joel@joelfernandes.org>,
        "quic_neeraju@quicinc.com" <quic_neeraju@quicinc.com>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] rcu: Add a warnings in sync_sched_exp_online_cleanup()
Message-ID: <20220627211658.GN1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220621012412.2724457-1-qiang1.zhang@intel.com>
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On Wed, Jun 22, 2022 at 11:43:26PM +0000, Zhang, Qiang1 wrote:
> 
> Add Cc 
> 
> Currently, the sync_sched_exp_online_cleanup() is invoked in
> cpuhp per-cpu kthreads when CPU is going online, so the CPU id
> obtained by get_cpu() should always be equal to the CPU id of
> the passed parameter, that is to say, the smp_call_function_single()
> never be invoked, if be invoked, there may be problem with cpu-hotplug,
> this commit add WARN_ON_ONCE() to remind everyone.
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>  kernel/rcu/tree_exp.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index be667583a554..ae8dcfd4486c 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -865,6 +865,8 @@ static void sync_sched_exp_online_cleanup(int cpu)
>  		put_cpu();
>  		return;
>  	}
> +
> +	WARN_ON_ONCE(my_cpu != cpu);

If we are going to add this sort of warning, why not instead add it
to rcutree_online_cpu()?

The reason the warning has not been present is the prospect of concurrent
onlining at boot time, which might have rcutree_online_cpu() invoked
from CPU 0 for multiple CPUs at boot.  However, the for_each_online_cpu()
loop has recently been removed from rcu_init().

But I would like to hear what others think.  Would such a warning
significantly help debugging?

							Thanx, Paul

>  	/* Quiescent state needed on some other CPU, send IPI. */
>  	ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
>  	put_cpu();
> -- 
> 2.25.1
>